1. Field of the Invention
The present invention relates to an acoustic wave device.
2. Description of the Related Art
Hitherto proposed acoustic wave devices include end surface reflection-type acoustic wave devices that utilize acoustic waves reflected by edges of substrates. In such an end surface reflection-type acoustic wave device, an acoustic wave excited by IDT (interdigital-transducer) electrodes provided on a piezoelectric substrate is propagated up to edges of the substrate and is reflected by the edges. The end surface reflection-type acoustic wave device is manufactured as follows, for example. First, a disc-shaped piezoelectric wafer and a supporting wafer whose coefficient of thermal expansion is smaller than that of the piezoelectric wafer are joined together, whereby a disc-shaped composite wafer is obtained. Subsequently, the piezoelectric wafer included in the composite wafer is sectioned into a number of rectangular areas each having a predetermined size, and IDT electrodes are formed on the individual rectangular areas. Then, the composite wafer is cut into pieces of the rectangular areas by using a cutting device such as a dicing saw. This provides acoustic wave devices each including a substantially rectangular-parallelepiped composite substrate in which a piezoelectric substrate and a supporting substrate are joined together, with IDT electrodes provided on the piezoelectric substrate. The composite substrate is used for the purpose of reducing the change in the size of the piezoelectric substrate that may occur with the change in the temperature and thus suppressing the change in the frequency characteristics of the acoustic wave device that may occur with the change in the temperature.
However, when the composite wafer is cut, chipping (chip) may occur on the cut surfaces of the piezoelectric wafer. Such chipping has been a cause for the increase in the number of spurious peaks (irrelevant small peaks occurring around resonance-related peaks). To suppress the occurrence of such spurious peaks, PTL 1 and PTL 2 employ the following manufacturing methods. The method according to PTL 1 includes the following steps: (1) providing cutting grooves around rectangular areas with a dicing saw before forming IDT electrodes, (2) polishing the surface of a piezoelectric wafer and removing chipping formed around the openings of the cutting grooves, (3) forming IDT electrodes, and (4) performing cutting with a dicing saw that is thinner than the cutting grooves and obtaining pieces of acoustic wave devices. The method according to PTL 2 includes the following steps: (1) forming IDT electrodes on each of a number of rectangular areas defined on a piezoelectric wafer included in a composite wafer, (2) making cuts along rectangular areas from the side of a supporting wafer by laser cutting to a depth that does not reach the piezoelectric wafer, and (3) applying a stress that cuts the supporting wafer, thereby obtaining pieces of acoustic wave devices.